I believe Image type column currently supports only .jpg, .png and .gif images.

However if you wish to post PDF’s for viewing by users ,you may wish to take a look at the following sample for sharing PDF documents to users. The app share the PDF docs as part of column type file.

appsheet.com - Marketing Docs - An app for a marketing team to access customer case studies on and offline. Marketing Docs - An app for a marketing team to access customer case studies on and offline. appsheet.com